Solder pastes are bonding materials prepared by dispersing a solder powder in a flux, commonly also designated as vehicle. Solder pastes are applied e.g. to a printed circuit board by screen printing using a stencil or to any metalized surface by dispensing through a syringe so as to deposit a suitable small amount of the paste on each area to be soldered and are subsequently heated in a furnace to melt the solder and perform bonding. This soldering technique is generally called reflow soldering. Various heating methods including infrared heating, laser heating, hot air heating, and hot plate heating can be employed in the reflow furnace. Solder pastes must have certain rheological properties suitable for screen printing or dispensing.
The flux or vehicle of a solder paste comprises at least one activator, which can remove oxide layers from the bonding surfaces and from the solder powder to allow good bonding. The flux further comprises components, which adjust the rheological properties of the paste. The major components of conventional flux are rosins and rosin derivatives used at a concentration of 40 to 70 wt.-%, and a combination of solvents used at a concentration of 20 to 50 wt.-%, each relative to the total weight of the flux.
One drawback of such conventional fluxes is the fact that a considerable amount of the rosin and other solid components of the flux remain on or in the vicinity of the resulting soldered joint as a flux residue. During reflow, many chemical reactions take place between rosin, other components of the flux, and metal oxides on the solder alloy surfaces. The non-volatile part of the reaction products and inert components of the raw materials remain as residue on the workpieces. Commercial no-clean solder pastes have residues of 6 to 7 wt.-%.
The residue, if remaining in a significant amount, leads to a bad appearance of the soldered areas and impairs the contact of the pins of an electronic part to be soldered with the solder paste. Furthermore, the flux residue may cause the insulation resistance between circuits of the workpiece to decrease due to moisture absorption by the flux residue or may cause the circuits to be broken or disconnected by the formation of corrosion products from the flux residue. Therefore, workpieces intended for use in electronic equipment for which high reliability is required have to be washed with a cleaner, after reflow soldering, in order to remove the flux residue remaining in the soldered areas.
Frequently, electronic equipment has to be sealed against moisture to increase reliability. Sealing is done by resin molding the entire workpiece. In such a case, the workpiece must be washed with a cleaner to remove any flux residue prior to resin coating or molding. Cleaner on the basis of fluorinated and chlorinated solvents have been successfully used to dissolve the rosin in the flux residue. However, the necessity to clean the workpieces prior to resin molding introduces an additional costly production step. Moreover, the use of these solvents is now regulated since their vapors cause ozone depletion in the atmosphere.
Therefore solder pastes with low flux residue, so-called no-clean solder pastes, have been developed in the past. U.S. Pat. No. 5,176,759 e.g., discloses a solder paste with minimized flux residue remaining after soldering which comprises a powdered solder and a flux in admixture. The flux comprises from about 5% to about 40% by weight of carrier components which comprise a rosin or a rosin derivative, an activating agent, and a thixotropic agent and from about 60% to about 95% by weight of a solvent. The solvent predominantly (more than 50% by weight) comprises a 2-alkyl-1,3-hexanediol having 1 to 4 carbon atoms in the alkyl group. The viscosity of 2-alkyl-1,3-hexanediol used in the solvent is low—typically in the range of about 323 cps at 20° C. In this patent the remaining residue is visually observed and assigned one of four grades 1 to 4. No percentages of remaining flux are given. The decreased flux residue remaining after reflow soldering with this type of solder paste causes no substantial problems in ordinary electronic equipment for which high reliability is required. However, the decreased residue solder paste is not completely satisfactory for present-day electronic equipment for which ultrahigh reliability is required.
U.S. Pat. No. 6,887,319 B2 discloses a residue-free solder paste that does not contain any rosin. The residue-free solder leaves little or no flux residue after reflow soldering and comprises a solder powder mixed with a rosin-free pasty flux. The flux comprises at least one solid solvent and at least one highly viscous solvent in a total amount of 30 to 90 mass %, in addition to at least one liquid solvent, all the solvents vaporizing at a reflow soldering temperature. The flux may further contain 0.5 to 12% of a thixotropic agent such as a fatty acid amide and 1 to 15% of an activator selected from organic acids and their amine salts, the thixotropic agent and activator vaporizing in the presence of the solvents while the solvents are vaporizing. This solder paste seems to be satisfactory from the amount of residue left after reflow soldering, but it is still not satisfactory with regard to homogeneity, stability against paste separation, and wetting requirement.
Thus, there is a continued need for a good wetting, highly stable and homogeneous low-residue solder paste with at most 2% of the flux remaining after reflow soldering. The flux residue should be compatible with further processing steps, even if molding process has to be performed, thus avoiding the necessity to remove the flux residue by washing.